Temperature control for electric heating



Aug. 7, 1945. w. RICHTER TEMPERATURE CONTROL FOR ELECTRIC HEATING FiledJune 21, 1945 Walther Richter INVENT OR.

ATTORNEY! Patented Aug. 7, 1945 TEMPERATURE CONTROL FOB ELECTRIC HEATINGWalther Richter, Whitefish Bay, Wis alsignor to A. 0. Smith Corporation,Milwaukee, Win a corporation of New York Application June 21, 1943,Serial No. 491,051

9 Claims.

This invention relates to temperature controls for electric heating, andmore specifically to the type of control designed to cut oil the supplyof energy upon a given energy consumption as in the successive electricheating of like objects to a predetermined temperature.

In most electric heating operations the greatest variable factorencountered lies in the large fluctuations in voltage of the supplysystem employed, since the heating effect varies as the square 01' thevoltage. A 10% rise in voltage will mean a 21% rise in heating effectfor the same amount of time.

Assuming that the articles to be heated are of substantially uniformshape and mass and that a constant voltage input can be provided, itwould be possible to employ a timer for cutting oil the heatingoperation for each article upon the elapse 01' a predetermined time toeflect a given heating each article. However, with the wide fluctuationsin voltage in most factories, it is impossible to obtain a satisfactorycontrol of temperature for the articles by merely timing the successiveheating operations. Furthermore, the ordinary variations in mass of thearticles being heated result in different input requirements for thesame.

One of the objects of the present invention is to provide a device forcutting oil the heating current that functions to proportion the time ofheating substantially inversely as the heating effect, i. e., inverselyto the square of the voltage.

Another object of the invention is to provide such a timer withcompensation at least partially for the variation in heat loss in thearticles due to the variation in time of heating.

Another object is to more accurately control the heating operations toobtain a heating of successive articles to a predetermined liketemperature.

The invention has been embodied in an apparam tus for controlling theinduction heating of bomb bodies for forging the same, and theaccompanying drawing illustrates this embodiment.

In the drawing, the single figure is a diagrammatic showingoi aninduction furnace'with its circuit and control mechanism and an articledisposed in the furnace for heating.

The furnace may be of the general type illustrated, in which a magneticframe or core I is disposed to receive a bomb body 2 or other article tobe heated. The energizing or primary coils 3 are supplied with currentfrom a suitable power transformer 4 which in turn receives current fromthe power station I. The article 2 serves as the secondary oi theinduction furnace and has currents induced therein 0! the article.

Since there are usually several furnaces supplied with power from thesame station 5 and in addition, a large number of other power consumingdevices in the same shop supplied from the same station, there is a widefluctuation of voltage in the line. When the voltage is relatively highin any one furnace the heating is more rapid and, conversely, when thevoltage is low the heating is less rapid. This rapidity of heatingvaries as the square of the voltage, other things remaining equal.

Likewise, the ordinary variations in size and mass of the article beingheated result in different energy consumption for different articles atany given voltage.

In timing the heating of successive articles the voltage and mass istaken into account by employing a watt-hour meter 8 connected as shownin the drawing. The meter 6 is of ordinary construction having arevolving disc I rotated in response to the energy passing through thecoils of t e meter. For this purpose, the two lower current coils 8disposed beneath the dis 1 and a central voltage coil 9 disposed abovethe disc 1 provide the motive power for driving the disc.

The coils l are connected in series in a line it containing a relativelyhigh constant resistance which eflect the heating II and connectedacross the lines I! and i3 leading to the primary coils 3 of theinduction furnace,

I. A voltage divider having resistors l4 and I5 is connected acrosslines l2 and I3, and coil 9 is connected across the resistor is.

It the resistors l4 and ii of the voltage divider were constant or ofconstant ratio, the voltage applied to coil 8 would always hear a fixedratio to the line voltage being supplied to primary coils 3 01 thefurnace. Hence both the voltage applied to coil 8 and the currentflowing through coils 8 will vary as the line voltage varies and willefiect driving of disc I at a speed corresponding to the square of thevarying line voltage.

The rotation oi disc I eflects the opening of cutout switch I in theline l3 supplying current to the coil I or the induction furnace. Thisis accomplished as shown by means of the gears l8 driven by disc I andin turn tripping a pilot switch I! in an auxiliary control circuit 20operating a solenoid II for controlling switch it. Any other suitablemeans may be employed for operating switch I from the rotation of discI. A photoelectric counter responsive to the rotation of disc I has beenemployed.

The watt-hour meter connected as shown compensates ior variations involtage in the line supplying the induction furnace with energy, asdescribed above. It also compensates within limits for the differentenergy requirements or diflerent size workpieces. This lattercompensation results from placing the current coils 8 in series with thefixed dummy resistance ll so that the energy being measured and whichdetermines the length of the heating cycle is that being consumed in theconstant resistance ll.

Should a relatively large workpiece be subjected to heating, andassuming a constant voltage, the current input to the piece would begreater thereby tending to compensate for the size or the piece. Asmaller workpiece would in turn have a higher resistance and result inless energy consumption over a given period or time, therebycompensating for its lower mass. By basing the operation meter 6 uponthe energy consumed by a dummy resistance, the time of heating isdetermined independent of the size or the workpiece. The variation inthe heating time cycle results solely from the fluctuation of voltage inthe line.

It the switch It were always operated to stop the supply of current tothe furnace after a predetermined energy consumption registered by themeter 6, it is possible that a variable heating eflect might result fromthe tact that the heat losses from the article 2 are determinedprincipally by the time involved. Where the heating time is long, thereis a greater total heat loss from the article than where the heatingtime is short. This means that when the heating current is shut on uponthe consumption or a predetermined amount of electrical energy by thedummy resistance II, the article may still have a different temperaturethan desired.

In order to more accurately control the temperature of the article andto compensate ior variations in the heat loss, the voltage divider isconstructed with automatically varying resistances. For this purpose,resistor H may be a non-linear resistance such as an incandescent lampwith a tungsten filament which increases in resistance as itstemperature rises. An increased voltage across the voltage divider willthen cause the voltage across the lamp to increase proportionally morethan that across the total divider.

This means that where the line voltage feeding the furnace increases sothat the rate of heating 0! article 2 goes up, making for less time forheat loss, the watt-hour meter will run raster than the normalproportion of theratio oi the square of the voltages, so that lessenergy will be supplied to the article. Where the line voltage decreasesbelow the voltage known to give the correct heating or the article, thevoltage across resistance M will decrease in a greater proportion thanthe total voltage, and the watthour meter will turn at a lesser speedthan that proportionate to the square of the voltage and thereby supplythe predetermined greater amount of energy to the article to compensatefor the larger heat loss resulting from thelonger time of heating.

The amount of compensation available can be increased by employing anon-linear resistance such as carbon filament incandescent lamps forresistor l5. Carbon filaments have a negative temperature coemcient and,therefore, would shift the voltage distribution even more to increasethe proportion of voltage appearing across the resistor ll.

The amount or compensation available can be decreased by employing oneor more smaller tungsten filament incandescent lamps in resistor I! tocounteract in part the compensation provided by resistor ll. 4

The temperature control of the present inven tion provides a relativelyaccurate means of determining the heating of the article, and itcompensates for differences in size of the workpieces, for thefluctuation of line voltage and for different heating time. The controlcan be applied in either electric induction or resistance heating.

Various embodiments may be employed within the scope of the claims.

I claim:

1. In a temperature control for electric induction and resistanceheating operations, the combination with the work load and currentsupply lines of a heating system, a wattmeter having its voltage coilconnected across the current supply lines and its current coils inseries with a constant resistor also connected across the current supplylines in shunt to the work load, the connection of said voltage coilbeing made to a tap of a voltage divider bridging the supply line with anon-linear resistance in one element thereof to change the ratio betweenthe voltage of said line and that applied to the coil of the meter sothat the voltage will divide between the elements of the divider to givethe voltage coil a higher proportionate voltage when the line voltage ishigh and a lower proportionate volt age when the line. voltage is low tothereby compensate for different heat losses in the work and provide forthe more nearly equal heating of each successive workpiece, and meansresponsive to the operation of said meter for stopping the heating cyclein a predetermined time as modified by voltage changes in the supplyline and substantially by varying heat losses in the workpiece.

2. In a temperature control for electric induction and resistanceheating operations, the combination with the work load and currentsupply lines 01 a heating system, a wattmeter having its voltage coilconnected across the current supply lines and its current coils alsoconnected across the current supply lines in shunt to the work load, theconnection of said voltage coil being made to a tap of a voltage dividerbridging the supply line with a non-linear resistance in one elementthereof to change the ratio between the voltage of said line and thatapplied to the coil oi the meter so that the voltage will divide betweenthe elements of the divider to give the voltage coil a higherproportionate voltage when the line voltage is high and a lowerproportionate voltage when the line voltage is low to thereby compensatefor different heat losses in the work and provide for the more nearlyequal heating of each successive workpiece, a constant resistor inseries with the current coils of the meter to eiTect operation of themeter independent of the ing load, a tungsten filament incandescent lampconstituting at least part of the resistance of one branch of saidvoltage divider to modify the voltage applied to said meter so that thevoltage will divide between the elements of the divider to give thevoltage coil a higher proportionate voltage when the line voltage ishigh and a lower proportionate voltage when the line voltage is low.

4. In a device of the class described having a working load, currentsupply lines, and a wattmeter with its voltage coil connected across oneelement of a voltage divider shunting the working load, a tungstenfilament incandescent lamp constituting a substantial part of theresistance of said element of said voltage divider to modify the voltageapplied to said meter so that the voltage will divide between theelements of the divider to give the voltage coil a higher proportionatevoltage when the line voltage is high and a lower proportionate voltagewhen the line voltage is low.

5. In a device of the class described having a working load, currentsupply lines, and a wattmeter with its voltage coil connected across oneelement of a voltage divider shunting the working load, a non-linearresistance constituting a substantial part of the resistance 01' oneelement of said voltage divider to modify the voltage applied to saidmeter so that the voltage will divide between the elements of thedivider to give the voltage coil a higher proportionate voltage when theline voltage-is high and a lower proportionate voltage when the linevoltage is low.

6. In a device of the class described having a working load, currentsupply lines, and a wattmeter with its voltage coil connected across oneelement of a voltage divider shunting the working load, a carbonfilament incandescent lamp constituting at least part of the resistanceof the-- other element of said voltage divider to modify the voltageapplied to said meter so that the voltage will divide between theelements of the divider to give the voltage coil a higher proportionatevoltage when the line voltage is high and a lower proportionate voltagewhen the line voltage is low.

'7. In a device of the class described having a working load, currentsupply lines, and a wattmeter with its voltage coil connected across oneelement of a voltage divider shunting the working load, a substantialpart of the resistance of said element having a positive temperaturecoefficient of electrical conductivity to modify the voltage applied tosaid voltage coil during the operation of said meter so that the voltagewill divide between the elements of the divider to give the voltage coila higher proportionate voltage whenthe line voltage is high and a lowerproportionate voltage when the line voltage is low.

8. In a device of the class described having a working load, currentsupply lines, and a wattmeter with its voltage coil connected across oneelement of a voltage divider shunting the working load, a substantialpart of the resistance of another element of the divider having anegative temperature coeiiicient of electrical conductivity to modifythe voltage applied to said voltage coil during the operation of saidmeter so that the voltage will divide between the elements of thedivider to give the voltage coil a higher proportionate voltage when theline voltage is high and a lower proportionate voltage when the linevoltage is low.

9. In a device of the class described having a working load, currentsupply lines, ahd a wattmeter with its voltage coil connected across oneelement of a voltage divider shunting the working load, the employmentof resistances in the opposite elements of said voltage divider havingdifierent temperature coeflicients of electrical conductivity to modifythe voltage applied to said voltage coil during the operation 01 saidmeter. so that the voltage will divide between the elements of thedivider to give the voltage coil 9. higher proportionate voltage whenthe line voltage is high and a lower proportionate voltage when the linevoltage is low.

WALTHER RICHTER.

